The Candidate Photoperiod Gene <i>MtFE</i> Promotes Growth and Flowering in <i>Medicago truncatula</i>.

Abstract

Flowering time influences the yield and productivity of legume crops. <i>Medicago truncatula</i> is a reference temperate legume that, like the winter annual <i>Arabidopsis thaliana</i>, shows accelerated flowering in response to vernalization (extended cold) and long-day (LD) photoperiods (VLD). However, unlike <i>A. thaliana</i>, <i>M. truncatula</i> appears to lack functional homologs of core flowering time regulators <i>CONSTANS</i> (<i>CO</i>) and <i>FLOWERING LOCUS C</i> (<i>FLC</i>) which act upstream of the mobile florigen FLOWERING LOCUS T (FT). <i>Medicago truncatula</i> has three LD-induced <i>FT-like</i> genes (<i>MtFTa1</i>, <i>MtFTb1</i>, and <i>MtFTb2</i>) with <i>MtFTa1</i> promoting <i>M. truncatula</i> flowering in response to VLD. Another photoperiodic regulator in <i>A. thaliana</i>, <i>FE</i>, acts to induce <i>FT</i> expression. It also regulates the FT transport pathway and is required for phloem development. Our study identifies a <i>M. truncatula FE</i> homolog <i>Medtr6g444980</i> (<i>MtFE</i>) which complements the late flowering <i>fe-1</i> mutant when expressed from the phloem-specific <i>SUCROSE-PROTON SYMPORTER 2</i> (<i>SUC2</i>) promoter. Analysis of two <i>M. truncatula Tnt1</i> insertional mutants indicate that <i>MtFE</i> promotes flowering in LD and VLD and growth in all conditions tested. Expression of <i>MtFTa1</i>, <i>MtFTb1</i>, and <i>MtFTb2</i> are reduced in <i>Mtfe</i> mutant (NF5076), correlating with its delayed flowering. The NF5076 mutant plants are much smaller than wild type indicating that <i>MtFE</i> is important for normal plant growth. The second mutant (NF18291) displays seedling lethality, like strong <i>fe</i> mutants. We searched for mutants in <i>MtFTb1</i> and <i>MtFTb2</i> identifying a <i>Mtftb2</i> knock out <i>Tnt1</i> mutant (NF20803). However, it did not flower significantly later than wild type. Previously, yeast-two-hybrid assays (Y2H) suggested that <i>Arabidopsis</i> FE interacted with CO and NUCLEAR FACTOR-Y (NF-Y)-like proteins to regulate <i>FT</i>. We found that MtFE interacts with CO and also <i>M. truncatula</i> NF-Y-like proteins in Y2H experiments. Our study indicates that despite the apparent absence of a functional <i>MtCO-like</i> gene, <i>M. truncatula FE</i> likely influences photoperiodic <i>FT</i> expression and flowering time in <i>M. truncatula via</i> a partially conserved mechanism with <i>A. thaliana</i>.